A current sense device senses magnitude of electrical current in an electrical circuit. Current sense devices are used in a wide variety of applications. For example, current sense devices are commonly used to determine current magnitude in power management applications, such as for over-current protection, current-mode control, power monitoring, and/or load-dependent voltage positioning.
Many current sense devices include a discrete current sense resistor, where current through the resistor generates a voltage proportional to magnitude of current through the resistor. The resistor voltage is measured to determine magnitude of current through the resistor. Although these current sense devices are relatively inexpensive, significant power can be dissipated in the current sense resistor, causing power loss and associated heat generation. Consequently, current sense devices including a discrete current sense resistor are not well suited for applications requiring high efficiency or for applications where heat generation is objectionable.
Some other current sense devices rely on parasitic resistance of an electrical circuit component to sense current magnitude. For example, some switching power converters use parasitic resistance of an inductor as a current sense element, where voltage drop across the parasitic resistance is measured to determine magnitude of current through the inductor. Although these current sense devices do not dissipate significant power, they are typically incapable of precisely sensing current magnitude due to variations in the parasitic resistance. For instance, parasitic resistance of an inductor may vary significantly among inductor samples, and the parasitic resistance may also vary significantly with inductor temperature.
Many power management applications include one or more transistors. For example, switching power converters typically include one or more transistors which repeatedly switch between their conductive and non-conductive states. Non-dissipative current sense devices have been developed to measure current through these transistors. For example, FIG. 1 illustrates a prior art current sense device 100 configured to determine magnitude of current IL through power transistor 102. Current sense device 100 includes a sense transistor 104, a differential amplifier 106, and a transconductance device 108. A drain (D) of sense transistor 104 is electrically coupled to a drain (D) of power transistor 102, and a source (S) of sense transistor 104 is electrically coupled to transconductance device 108 at a node Vy. Inverting and non-inverting inputs of differential amplifier 106 are electrically coupled to nodes Vy and Vx, respectively, and a source (S) of power transistor device 102 is electrically coupled to node Vx. Differential amplifier 106 drives transconductance device 108 to generate current Io through sense transistor 104 such that voltage at node Vy is equal to voltage at node Vx. It can be determined that current IL is related to current Io as follows, where Rp is on-resistance of power transistor 102 and Rs is on-resistance of sense transistor 104:IL=IoRs/Rp  (EQN. 1)
Gate length, channel doping, and gate oxide thickness of sense transistor 104 matches that of power transistor 102 so that sense transistor 104 has a similar threshold voltage and current density to power transistor 102. Sense transistor 104 has an on-resistance that is known multiple of an on-resistance of power transistor 102. Consequently, a ratio of Rs to Rp is known, and current IL can be determined from current Io using EQN. 1 above.